In power supplies for airframes, multiphase power supplies are used which have a first multiple phase power source which supplies electrical power to electrical loads, which require a minimum of interruption of power, and a second backup power supply which is connected to the electrical load upon the detection of an electrical fault in one or more of the phases of the first power source. These power supplies do not disconnect the phases of the first power source which do not have a fault at a precise point during the cycle of the AC power provided by the first power source and do not control the connection of the phases of the second power source to the electrical load at a precise point during the cycle of the phases provided by the second power source.
Power supplies are known in which a first operating power supply is backed up with a second power supply which is connected to the electrical load upon the detection of a fault condition in the first power supply. See U.S. Pat. Nos. No. 3,337,742, 4,075,502, 4,087,697, 4,096,395, 4,231,029, 4,384,213, 4,405,867, 4,520,275 and 4,583,004. None of these power supplies disconnects the phases of the first power supply which do not have a fault at a precise point during the cycle of the AC power provided by the first power source and controls the connection of the phases of the second power source to the electrical load at a precise point during the cycle of the AC power provided by the second power source.
It is known that switching losses and stress to a switch may be minimized by switching a switch when the voltage drop across the switch is zero. U.S. Pat. Nos. 4,138,715, 4,196,469 and 4,413,313 disclose that switching losses from a transistor may be minimized by switching the transistor state during the time that the voltage across the transistor is zero.